This invention relates to marine seismic exploration and more particularly to the determination of reflectivity and water depth for use in firing a linear array of sources as an inverse distortion operator.
In marine seismic exploration, seismic energy can be generated from a towed array of sources. The resultant acoustic pressure waves are reflected from the subsurface formations and recorded as seismograms. Unfortunately, the seismograms include distortion which obscures the characteristics of the subsurface formations. In marine exploration, this distortion is often severe. It includes water bottom multiples as depicted in FIG. 1 and reverberations as depicted in FIGS. 2 and 3.
As the initial shot pulse travels to the bottom of the water, it is partially transmitted and partially reflected at this boundary. The transmitted part is reflected by the discontinuity beneath the water to become a primary reflection. The reflected part bounces back to the surface where it is again reflected to the water bottom. Once more, part of this pulse passes through the boundary and part is reflected. This process continues indefinitely. Each time a pulse bounces between the surface and the bottom of the water, a pulse emerges from the boundary at the bottom to follow the initial pulse into the section. These multiple pulses are delayed equal time intervals relative to each other, and their amplitudes are successively reduced by the reflection coefficient at the bottom of the water. The net result of this is to produce a seismogram consisting of a single primary followed by a train of equally spaced multiples or reverberations.
In U.S. Pat. No. 3,689,874 to Foster et al there is described a method and apparatus for removing the reverberation distortion present in seismic data by an operation which separates the reverberation distortion component of the data from the component representing the characteristics of the subsurface formation. This operaion is known as dereverberation and more particularly involves the obtaining of a dereverberation operator in the form of an inverse filter that is applied to the seismic data to produce a signal which approximates the signal which would have been produced by a desired shot pulse interacting with the reflectivity of the subsurface formation in the absence of the reverberation distortion.
While dereverberation has been successfully used on many marine seismograms, it is not effective in many areas. Particularly where the water bottom is hard, the reverberations are so severe that the complete dynamic range of the recording system is required just to faithfully record the reverberations. Accordingly, it is desirable to suppress the reverberations before recording.